The present invention relates to an air separation process and an air separation plant. In particular, it relates to a process which produces a nitrogen-enriched stream at a pressure of at least 2 bar which is expanded in a turbine.
In particular, it relates to an air separation process and an air separation plant which are integrated with a combustion chamber.
Cryogenic air separation units conventionally operate with two distillation columns, one called a medium-pressure column, operating at about 4 to 10 bar, and one called a low-pressure column, operating at between 1 and 3 bar.
An increase in these pressures, although making the distillation more difficult, would be beneficial, as it would allow the volume of the equipment (and therefore their costs) to be reduced and would allow energy irreversibilities due to head losses in the various circuits to be reduced.
However, it is quite rare to be able to increase these pressures, as it is necessary to utilize for economic purposes the energy contained in the waste fluids not conventionally xe2x80x9ccommercially utilizablexe2x80x9d because of their purity levels.
The conventional solutions are, for example:
to reinject this waste into gas turbines (in particular the case of IGCC plants);
to subject this fluid to cold expansion in a turbine so as to produce liquid;
high-temperature expansion in a turbine (as described in patent application EP-A-0 402 045). DE-A-2 553 700 describes an air separation unit which produces a nitrogen-enriched gas stream. After a compression step, the gas stream is heated by indirect heat exchange inside a combustion chamber before being expanded in a turbine. The gas expanded in the turbine serves to preheat the compressed gas to be sent to the combustion chamber.
U.S. Pat. No. 3,950,957 discloses an air separation unit in which the nitrogen produced is expanded after being heated up in a boiler. The remaining heat in the expanded nitrogen is transferred to the boiler by indirect heat exchange.
In U.S. Pat. No. 5,459,994, a nitrogen stream is expanded in a turbine, mixed with air, compressed and sent to a combustion chamber.
In U.S. Pat. No. 4,729,217, after having been mixed with the fuel, the nitrogen is expanded in a turbine and sent to a combustion chamber.
U.S. Pat. No. 4,557,735 describes the case in which the nitrogen is expanded at a cryogenic temperature, compressed, mixed with air and sent to a combustion chamber.
EP-A-0 959 314 relates to a process for expanding a mixture of air and waste nitrogen, in which the mixture is sent to a combustion chamber.
The proposed scheme corresponds to the waste nitrogen undergoing expansion in a turbine at high temperature in an innovative and effective manner.
It is one object of the invention to provide an air separation process in which a stream of compressed and purified air is separated in an air separation unit in order to produce a nitrogen-enriched gas stream at between 2 and 7 bar, the nitrogen-enriched gas stream is expanded in a turbine and the expanded gas stream is sent to a convection region located downstream of a combustion chamber, characterized in that the gas stream is expanded without having been mixed with a stream of fuel and it is not mixed with a stream of air after its expansion.
Optionally:
the nitrogen-enriched gas stream is preheated by indirect heat exchange with the gases inside the combustion chamber before being expanded;
the temperature at which the nitrogen enters the turbine is at least 700xc2x0 C.;
the nitrogen-enriched stream is preheated by indirect exchange in the combustion chamber in one step up to an intermediate temperature and then in a second step up to the turbine entry temperature and the expanded gas sent into the combustion chamber gives up heat to the gas stream to be expanded during the first preheating step;
the nitrogen-enriched gas stream is compressed to a pressure of between 5 and 20 bar before being expanded;
the air is cooled after its compression by means of an absorption refrigerating unit and pressurized water intended for the refrigerating unit is heated by the gases from the combustion chamber to which gases the nitrogen-enriched gas stream is added;
the air is purified in a purifying means before being sent to the separation unit, the purifying means is regenerated by a nitrogen-enriched gas stream and at least one portion of the stream that has served for the regeneration is sent to the expansion turbine;
the nitrogen-enriched stream is withdrawn from a single column or from the medium-pressure column and/or the low-pressure column of a double column or from the high-pressure column and/or the intermediate-pressure column and/or the low-pressure column of a triple column;
the nitrogen-enriched stream is mixed with a nitrogen-enriched gas coming from an external source before being expanded in the turbine;
the nitrogen-enriched stream contains at least 50 mol % nitrogen and between 0.5 and 10 mol % oxygen;
the column from which the nitrogen-enriched stream comes operates between substantially 2 and 7 bar;
the nitrogen-enriched stream is not mixed with air before being expanded in the turbine;
a nitrogen-enriched stream, preferably containing at least 50 mol % nitrogen, coming from an external source, is mixed with the nitrogen-enriched stream coming from the air separation unit, upstream of the expansion turbine.
Another object of the invention is to provide an air separation plant comprising:
i) an air separation unit operating by cryogenic distillation,
ii) a combustion chamber followed by a heat-recovery region comprising a convection region,
(iii)an expansion turbine,
(iv) means for sending air to the air separation unit operating by cryogenic distillation,
(v) means for withdrawing a nitrogen-enriched gas from the air separation unit operating by cryogenic distillation,
(vi) means for sending the nitrogen-enriched gas to the expansion turbine and
(vii) means for sending the nitrogen-enriched gas from the expansion turbine to the convection region located downstream of the combustion chamber
characterized in that it comprises neither means for mixing air with the nitrogen-enriched gas downstream of the turbine and upstream of the combustion chamber nor means for mixing fuel with the nitrogen-enriched gas before its expansion.
Optionally, the plant may comprise:
means for preheating the nitrogen-enriched gas stream by indirect heat exchange with the gases inside the combustion chamber upstream of the expansion turbine;
means for preheating the nitrogen-enriched stream by indirect exchange in the combustion chamber in one step up to an intermediate temperature and then in a second step up to the turbine entry temperature;
a refrigerating unit in which the air is cooled after it has been compressed, a pressurized-water circuit intended for the refrigerating unit and a means for heating the pressurized-water circuit by the gases from the combustion chamber, to which gases the nitrogen-enriched gas stream has been added;
a purifying means in which the air is purified before being sent to the separation unit, the purifying means being regenerated by a nitrogen-enriched gas stream, and means for sending at least a portion of the stream that has served for the regeneration to the expansion turbine;
means for withdrawing the nitrogen-enriched stream from a single column or from the medium-pressure column and/or low-pressure column of a double column or from the high-pressure column and/or the intermediate-pressure column and/or the low-pressure column of a triple column; and
means for mixing a nitrogen-enriched waste gas (preferably containing at least 50 mol % nitrogen) coming from an external source with the nitrogen-enriched gas to be expanded.